Method and apparatus for measuring beam current



Oct. 28, 1952 M. G. vHOLLOW/w METHOD AND APPARATUS FOR MEASURIG BEAM CURRENT 4 smanats-shmaty 1 Filed Nov. '7. 1945 l INVENToR. Marshall G Holloway' Oct. 28, 1952 M. G. HoLLowAY 2,616,053

METHOD AND APPARATUS FOR MEASURING BEAM CURRENT Filed Nov. '7, 1945 4 Sheets-Sheet 2 www /f/VL'JJS IN1/Ely TOR.

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METHOD AND APPARATUS FOR MEASUR- ING BEAM CURRENT Marshall G. Holloway, Ithaca, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Application November 7, 1945, Serial No. 627,268

3 Claims.

This invention relates to an improved method and means for making nuclear measurements, and more particularly to a method and apparatus for measuring the various phenomena, capable of electronic detection, produced by the accelerated charged particle beam in a cyclotron.

In the disintegration or transmutation of various elements by bombardment by high speed charged particles, the determination, for example, of the cross-section of the reaction (i. e. probability that the reaction will occur) is an important consideration. In other words, it is important in the investigation of nuclear reactions, to determine how many transmuted atoms will result from a given type of bombardment for` a predetermined period of time. The determination of such cross-section from observed experimental data requires, inter alia, the measurement of the number of incident bombarding charged particles, and the energy distribution of them.

Many methods have been described for determining the energy and intensity of the incident beam in a cyclotron and other accelerating devices. Pulse amplifiers of varied sensitivity have been employed in the art which are arranged together with counting circuits to permit more or less accuracy in such measurements. Similarly the radioactivity induced in a substance has been determined and the number of bombarding particles necessary to produce such radioactivity can be estimated. However, all of the methods previously used in the art have employed only approximate calibration techniques in order to determine the energy and the intensity of an accelerated beam of charged particles. This was particularly true where, because of the requirements of many transmutation operations, the current value of the beam of bombarding particles was too large to be counted by a pulse amplier and counting circuit alone, and yet too small to be measured by other more conventional methods such as, for example, milliammeters and/or the like. c

It will be seen, then, that this invention has as an object, to provide a method for rapidly determining the energy of the particles of an accelerated charged particle beam.

It is another object of this invention to provide means for precisely determining the number of charged particles in an accelerated beam of such particles.

It is a further object to provide a simple compact means for the measurement of beam currents .and energies in an apparatus for the acceleration of charged particles such as a cyclotron. Y

Other objects will be apparent to one skilled in the art from the following detailed description and illustrative examples:

In accordance with the objects stated andin ionization caused by said accelerated chargedj particles; and calibrated electronic circuit means alternately connected to the electrode to permit discrimination between the ionization current caused by charged particle bombardment of a gas in the target zone and the pulses caused by said ionization bursts detected by said electrode.

In order to illustrate the advantages of the present invention and to explain the attainment of the objects above stated, reference is made to an embodiment used in connection with a cyclotron type of charged particle accelerating device although it will be apparent that with only minor changes and variations, the invention can be adapted to many other types of accelerating apparatus. Such an embodiment and various experimental data curves are shown in the attached drawings which are made a part of this specification in which:

Figure l is a schematic view in plan of a portion of a cyclotron structure indicating the location of the various elements with respect to the accelerated charged particle beam;

Figure 2 is an elevational view taken on the line 2--2 in Figure 1, partly in section and partly cut away, showing the location of the probe electrode and details of its construction;

Figure 3 shows further details of the insulating, air-tight support for the probe electrode;

Figure 4 is a chart showing typical energy distribution curves as obtained by the use of the continuous loop probe electrode with a pulse amplifier and Figure 5 is a chart of the typical relative ionization caused by a charged particle beam at various incident energies.

Referring to the drawings and more particularly, Figure 1, a cyclotron structure indicated at IB, having accelerating electrodes, gate valves, sealed iiexible mountings and like accessories, and

in which the beam direction is indicated by the 3 arrow, is provided with a sealed air-tight disintegration chamber II, cylindrical in form which surrounds and extends outwardly of the exit port or exit aperture plate I2 of the cyclotron.

The subatmospheric state of the cyclotron is maintained by a thin aluminum sealing exit foil I3, vhich permits the penetration of charged particles therethrough, soldered or otherwise attached in air-sealing relationship over the exit aperture in the port plate I2. Support for the disintegration chamber I I, is provided by the annular flange member I4, which is made integral with the exit tube I5, of the cyclotron by a sealed joint such as is attained by brazing or soldering and includes the reinforcing ring I6.

Various collimating elements I I, and collecting plates I8, for disintegration particles.. areprovided in the disintegration chamber, but, because they do not form part of the present invention, their construction will not be given in detail. The function of these elements is to permit the determination of various factors in a typical J cross-section investigation such as will be discussed later, and more particularly from the standpoint of the present invention they serve to define the target zone in which disintegration and/or transmutation are effected. The collecting plates are connected to a suitable pulse ampliying and recording circuit, many types of which are well known in the art, and are. indicated schematically at |53.

The probe electrode 26, which is one of the features of the present invention, is positioned within the disintegration chamber II, in spaced relationship with the exit foil I3, thus being positioned between the exit aperture and the target zone donned by the collecting plates I8, and the collimating elements I'I. The electrode comprises a mil copper wire bent to a U shape and having its respective ends inserted into respective accommodating recesses in the copper stand-ori supporting member` 2l (Figure 2) and rigidly fixed therein by soldering or the like so as to form a continuous elongated loop. As shown in Figure 2, the Width of the loop is slightly greater than the width of the exit aperture in the port I2. Furthermore, the length of the loop electrode is determined and the electrode positioned so as not to. obstruct the beam and prevent the desired bombardment of the target element in the. target zone.

The support member 2-I is rigidly mounted on the flange I4 and insulated therefromV by means of the arrangement shown in Figure 3, though any equivalent structure would be satisfactory if means for permitting an insulated air-sealing structure are provided. Referring to Figure 3, it will be noted that the copper mounting stud 22 is threaded at both ends and is provided with an integral large diameter portion 23, which is adapted to press against a continuous annular gasket 24 maintained in collar 25. The electrically insulated relationship between the support 2l, together with the studA 22, and the member I4, is provided by the insulating bushings 25-and 26 adapted to accommodate the stud 22 and to position it in the aperture in the member I4. The bushings 25 and 26 may be of Lucite or similar insulating materials which may be readily machined to facilitate manufacture. In this embodiment, annular recesses are machined in both faces of collar 25 to accommodate gaskets 24 and 2'I so that upon assembly of the mounting the sealed state of the chamber i I can be maintained. Bushing 26 has only a single annularl recess machined therein to accommodate gasket 2B. To assemble the insulating mounting, the gaskets 21 and 28 are placed in the recesses provided in insulating bushings 25 and 26 respectively. The bushings are positioned in the accommodating aperture drilled in member I4 and stud 22 is passed'through the central bores in each of the bushings so that the flanged portion 23 presses against gasket 23. Nut 29, when drawn up on the threaded end of the stud extending outwardly ci the chamber, draws the component parts into airsealed assembly. The electrode support 2I is then mounted on the end of the stud, which extends inwardly of the disintegration chamber and is fixed in position by tightening nut S.

It will thus be seen that the stud 22 acts as a feed-through conductor of the pulses and/or ionization currents detected by the probe electrode 20, during operation, and passes in an insulated and air-sealed manner through the walls of said chamber to permit such pulses and/or ionization currents to be amplied and recorded through the use. of suitable electronic circuits indicated at 3l and 32 (Figure l). The lug 33., held in conducting relationship with stud 22, by suitable means such as a nut or the like, indicates the continuity of the circuit from the feed-through conductor through the manually operated selector switch 34, to. the alternate electronic cir-u cuits 3I and 32, discussed hereinafter.

Circuit 3l, hereinaiter referred to as the plie-- tron circuit as it utilizes an electronic tube known in the art as a pliotron or direct current amplifying tube, is of standard typev and will not be dis.- cussed in detail. Any circuit which is responsive to extremely small currents and is capable of amplifying those currents to give readings on standard meters or to actuate suitable standard recording device, may be employed. In the particular disintegration operation described by way of illustration hereinafter, it was found that a` bal,- anced type of circuit using a Western Electric 96457 vacuum tube was satisfactory. Such a circuit is shown, for example, in the Review of Scientific Instruments, volume 6, page et seq., 1935.

The ionization current detected by the electrode 2B produces a voltage drop across a xed value precision resistor selectively inserted in the circuitV by a suitable selector switch (not shown) and predetermined to permit the greatest degreeof stability possible. This'voltage drop is measured by the pliotron.

Circuit 32 includes a linear pulse amplifier ofconventional design. The output of the amplifier is split-into two channels which are made as nearly alike as possible. Each of the channels has a scale of thirty-twoy connected to the output.

of a biased thyratron. It is thus possible t0 bias the output of one thyratron to respond to smallv pulses and to monitor. operation of the` other` The distance between the extended straight portions of the loop electrode 20 is about vthreeeighths of an inch to minimize. interference with the operation of the collecting elements. IB and with bombardment in the target zone and. yet is positioned close enough to the boundaries of' the incident charged particle beam to detect the ion'- ization caused thereby as well as-tobeiresponsive to pulses therefrom. The electrode 20 is spaced about one-eighth of an' inch from the port plate l2, although this dimension may be varied depending on the particular application.

A screen 35, is movably supported in the exit tube I5, in advance of the exit port plate I2 (see Figure 1). This screen may be of. very ne meshed wire construction or any `other .type found suitable and is capableA of reducing the beam intensity by a known amount, as for example, by a factor of ten, in order to facilitate calibrationrof the pliotron as will be explained later. The screen is mounted on a rotatable support indicated at 36 in Figure 1, and extends through any well-known vacuum seal 31, such as a Wilson seal for example, to the outside of the exit tube I5, of the cyclotron. .The support structure 36, is arranged so that rotation thereof causes the screen, 35,` to be removedvfrom the, path of the accelerated charged particle beam.

Now, a typical operation of the illustrative ernbodiment describedabove occurs in the determination of the cross-section at various incident energies for the reaction resulting from the bombardment of deuterium with ionized particles of the helium isotope of mass three which may be written in the form A source of ionized helium particles i-s provided at the center of the cyclotron I0, and acceleration of these chargedparticles is accomplished in the standard manner now well-known in the art. The deuterium target in gaseous form, is introduced into the disintegration chamber II. The problem then, in the determination of the cross-section is to determine thenumber of target atoms (i. e. deuterium atoms) and the yield in helium 4 or alpha particles. Y l

The determination then, involves the evaluation of the relationship in which 6a. is the absolute cross section at any particular energy averaged for the thick target used and the energy distribution of the incident beam: Nt is the number of effective target atoms per square centimeter of cross section of the beam and E is the average solid angle in the center of mass system in which disintegration alpha particles are observed. Y is the yield, that is, the number of alpha particles, divided by the number of incident deuterium particles, and is obtained by iirst calibrating the pliotron circuit f in the following manner. The screen 35 is interposed in the cyclotron charged particle beam, thereby reducing the intensity thereof to a point where the resolution time of the linear pulse amplifier circuit 32 permits accurate counting and a count of the number of incident particles per unit time (designated as n) in the target zone is made with manual selector switch 34 moved to the appropriate position to connect probe electrode 20 to pulse amplifier circuit 32. Without altering the beam intensity, switch 34 is then moved to connect probe electrode 20 to the pliotron circuit 3| and a current reading proportional to the ionization current caused by the incident particles is taken from the meter provided in the plate circuit of the pliotron, but not shown. This current reading which is proportional to the voltage drop across input resistor of value R1 is designated as i. A relationship is thus arrived at between the number of Zone.

incident particles per unit time, and the meter reading (i. e., the ionization produced in ythe target zone by the incident particles). y

When screen 35 is removed by rotating support 36, the undiminished beam enters the vtarget Though the counting rate necessary to permit an accurate count to be made is higher than obtainable with present equipment, the number of incident particles may still be determined from the meter reading (designated-by I). which is proportional to the ionization caused by the undiminished beam. This current reading is proportional to the voltage drop across an input resistor R2 in the pliotron circuit. It is apparent then, that if the resistor ratio R2/R1 is designated by R that the number of incident particles per unit time in the undiminished beam (N). may be arrived'at from the proportion.

By defining a pliotron calibration number P as the ratio of the number of incident particles per unit time to the meter reading they produce, P is then equal to I or iR The yield Y, is then determined by counting the disintegrations per second (D) as alpha particles detected by the pulse amplier circuit I9 through electrodes I8, and evaluating the relationship:

D gg N- In Inasmuch as the number of target atoms Nt is readily determined fro-m the temperature and pressure of the gas at the time of filling and the dimensions of the target chamber, it is clearthat the novel methods here described largely simplify cross section determinations.

Due to the fact that the cross section ofthe reaction varies with the incident energy, a determination of this factor is necessary which emphasizes further utility of the system disclosed herein. The pulse amplier circuit includes suitable standard electronic devices or the like such as a recording oscilloscope or oscillograph -for indicating and/or recording the pulse heights detected by electrode 20. It is thus possible to compare the pulse heights from known-energy range particles, such as from alpha particles emitted by polonium to obtain the energy distribution of the incident particles. Figure 4 shows two such distribution curves prepared from data obtained according to the above method.

The calibration of the pliotron may be extended from the base energy at which the value of the equation PTR,

is determined, by the preparation of a relative ionization curve such as is shown in Figure 5. This curve is obtained by varying the incident energy through the use of a series of suitable foils not shown in the drawings. A satisfactory type would be one described in the copending U. S. patent application of Charles Baker, S. N. 610,644, led August 13, 1945, now Patent No. 2,485,470. At a particular incident energy as determined by the pulse heights recorded by the pulse amplifier circuit, the ionization current is indicated by the pliotron. Now, the pliotron calibration, as has been noted, is dened as the ratio of the number of-.particles 'to thefionization current observed, and therefore the pliotroncalibration is inversely proportional -to the relative ionization current l,as given .by .the relative ionization Vcurve `(Figure 5). Hence,

Where 'Pc and lo are .the direct plietron calibrationand the maximum relative 'ionization respec tively and .P and I are the V.eorresponding quan titiesfatrany other energy.

While a preferred embodiment of the invention has been given .and a particular disintegration operation indicating the general usefulness of -a probe electrode positioned outwardly of the exit foil and in advance of the target'zone .of a v.cyclotron :type of charged .accelerating device, has been described by Way of illustration, it is obvious that many variations in use, structure and the methods described, are possible. The shape of the electrode and the position with respect to the -beam are governed 'by factors which vary depending on the disintegration or transmutation operations being undertaken. Similarly, the particular electronic indicating and/or recording devices employed connection with the electrode and the activating circuits thereof may also be of types which record or lindicate other phenomena than are described above. vIt is there- "a fore apparent Athat -many changes in materials, structure and/or operation may be made without departing from the spirit and scope of the invention as defined in the appended claims.

It is claimed:

1. The combination with a charged particle accelerating device having an exit aperture to permit the emergence .of the charged particle beam therefrom, vand a target zone, of acontinu.-

ous loop electrode, means supporting said electrede between said exit aperture and said target zone and in a position to permit the emergent charged particle 'beam to pass through the zone defined by said loop, and electronic indicating means electrically connected to said electrode and to van equipotential point on said accelerating device whereby it is responsive to the phenomena detected thereby.

2, The combination with an enclosed-charged particle .accelerating vdevice having an accelerating zone, an exit aperture to permit the .emer-Y gence of the charged par-ticle beam therefrom, and a target zone, of a continuous loop `electrode, means supporting said 4electrode between said exit aperture and isaid target zone and in position to permit the Iemergent charged particle beam to pass through the zone defined by said loop, electronic indicating means including a direct current iamplier responsive to the ionization .caused by said emergent beam and detected by said electrode, ,conducting 'means electrically connecting said electrode, and conducting means electrically vvconnecting an equipotential point Aof saidacceleratingdevice :to lthe input of said cieca treme means.

l3. lThe combination with an enclosed charged particle accelerating v device'fhavingan accelerata ing zone, an .exit aperture to .permit the emere gence of the charged particle Ybeam therefrom, and a target zone, of a continuous loop electrode. means supporting saidelectrode between said exit aperture and said target zone and in lposition -to perm-it the emergent charged particle beam Vto pass-throughfthe zone defined by said loop, VVelectronic indicating lmeans including a linear pulse amplier circuit ,responsive to the pulse heights of the emergent charged particle beam and the number of 4,said particles as 4Adetected vby said electrode, and a plurality .of conducting means electrically .coupling said .electrode and an equipoten.- tial point of said particle accelerating Adevice with said `electronic means.

MARSHALL G, HOLLOWAY.

.REFERENCES 'QITrED The following references are of vrecord in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,948,384 Lawrence Feb. 20, 1934 2,097,860 Failla Nov. 2, 1937 FOREIGN lPATENTS Number .Country Date .252,207 .Great A:13T-itam ug-c- Aue. 4, ,1.9.27 

